A suspected arson attack on an animation production company in Japan killed 24 people and injured dozens more on Thursday, with flames gutting the building in the city of Kyoto.

Police said the fierce blaze appeared to have been started deliberately but there was no immediate information on a possible motive.

The toll continued to climb hours after the fire began, with fire department officials saying bodies were being discovered as they searched the ravaged building.

A fire department official told AFP that at least 11 more people had been found "in cardio-respiratory arrest," a term used in Japan to signify a victim's death before it is officially certified.

The discoveries, on the building's second floor and a stairwell leading to the roof, raised the toll to at least 33 dead.

Officials said 35 people had also been injured in the fire, 10 of whom were in serious condition, and local media said around 70 people were believed to have been in the building when the fire started.

Footage of the blaze showed thick white smoke pouring from the windows of the three-storey building. Its facade was charred black on much of one side where the flames had shot out of the windows.

Japan's Prime Minister Shinzo Abe took to Twitter to express his horror.

"It's so dreadful that I'm lost for words," he wrote.

"I pray for those who passed away."

The fire department said it began receiving calls around 10:35 am about the fire at the studio belonging to Kyoto Animation. 

"Callers reported having heard a loud explosion from the first floor of Kyoto Animation and seeing smoke," a fire department spokesman said.

Police said they were still investigating the cause of the fire but it was a suspected arson attack.

"A man threw a liquid and set fire to it," a Kyoto prefectural police spokesman told AFP.

Public broadcaster NHK reported that a man had been detained in connection with the blaze and was later taken to hospital for treatment.

It reported that the suspect had poured a gasoline-like substance around the building and said "drop dead" as he set fire to it.